chore: stricter control flow syntax validation in runes mode

[dummdidumm]

Handle whitespace characters between { and # / : / @ but disallow that in runes mode

closes #11975

While implementing this I noticed that the else if block AST is too ambiguous right now. Is a fragment with a sole if block an else if or a if? it's indistinguishable, which is bad for formatting tools like prettier. We should change it to the if block AST being directly placed inside the alternate branch. Will do that if others agree as a follow-up once this is merged.

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[Rich-Harris]

Is a fragment with a sole if block an else if or a if? it's indistinguishable, which is bad for formatting tools like prettier

Not sure I understand what you mean here, can you elaborate? FWIW these are different (with respect to transition locality):

{#if foo}...{:else}{#if bar}...{/if}{/if}

{#if foo}...{:else if bar}...{/if}

[dummdidumm]

I forgot there's the elseif property - ignore me 😅